1. Field of the Invention
This invention relates to both a method for producing a durable non-stick workpiece article and the article of manufacture produced by the inventive method.
More particularly, this invention relates to the consolidation of diamond powder to a binder coating the surface of a metal substrate.
2. Discussion of the Background
Non-stick articles are quite prevalent in today's world. Polytetrafluoroethylene (TEFLON) is one material that is widely-used for many non-stick applications. Besides TEFLON, silicone coatings are also used in non-stick applications. Both TEFLON and silicone, however, are very soft materials and suffer from abrasive wear which shortens the lifetime of the non-stick article. In addition, TEFLON and silicone have relatively low temperature ratings. For example, when a TEFLON coating becomes too hot, the TEFLON begins to decompose spontaneously.
Porcelain enameling is a commercial, industrial scale technology which is used to coat a variety of objects of various sizes and shapes. Porcelain by itself can provide a smooth, stick resistant surface. The uses of porcelain enameling include glass-lined water heater tanks, cookware, kitchen appliances, barbeques, heat exchangers, architectural panels, decorative jewelry, road signs, and silo panels used for grain storage. Porcelain enamel is used as a protective coating on a variety of metals and is used as a glazing on glassware and whiteware.
The porcelain coating industry for years has developed appropriate enamels for various base substrate materials. The porcelain coating industry relies heavily on experiential data to obtain the best enamel coating and adhesion to the wide variety of metals which it coats. The methods used to coat aluminum with porcelain are different than the methods used to coat steel with porcelain. Likewise, methods for coating low-carbon steel are different than methods for coating cast iron, and methods for coating copper and gold are different from methods for coating steel formulas. Methods of applying a porcelain coat to metal typically include the application of ground coats (also called base coats). Also, these methods typically contain top coats which will control the background color and finish of the coated workpiece; however, porcelain surfaces are prone to scratching, particularly when being cleaned with an abrasive.
Diamond, due to its low coefficient of friction and high hardness, is an excellent non-stick material; however, high-pressure, high-temperature (HPHT) synthesis of diamond on common metal and glass surfaces is not possible because the temperatures attained during HPHT synthesis exceed the melting points of many common metals and glasses. Further, HPHT synthesis is only practical for limited areas because of the high pressures (100 GPa) required for HPHT synthesis.
Diamond is known to have many superior properties when compared to other materials. These properties make diamond a viable candidate for applications where the surface of a material is exposed to mechanical, chemical, or physical wear. For example, diamond is known to have a low coefficient of friction and superior hardness. As such, diamond would be an ideal candidate for non-stick surfaces and could potentially replace TEFLON coatings on cooking surfaces. Diamond is also known to have low sputter yield and is extremely chemically resistant. As such, diamond would be an excellent protective coating on electrodes and surfaces exposed to plasmas and ion bombardment. Diamond's superior chemical resistance would make it an ideal container coating in applications where leaching of the wall materials by the chemical effluent either produces contamination of the process stream or produces a long-term reliability problem for the storage of the effluent.
Unfortunately, the application of diamond directly to typical surfaces presents high technical and cost barriers which have either made diamond application impossible or extremely costly. On the one hand, high pressure synthesis is used to produce nearly 100 tons of synthetic grit per year at a fairly cheap cost per carat (current selling prices are under $1.00/carat). The high temperatures and pressures required for high pressure synthesis of diamond are not compatible with coating diamond directly onto common materials such as steels, aluminum, or glassy materials. On the other band, chemical vapor deposition (CVD) of diamond is a viable technical approach for the formation of diamond on these common materials. Indeed, deposition on these materials directly or with the use of surface pretreatments or inter-layers has been demonstrated. U.S. Pat. No. 5,686,152 to Johnson et. al. teaches a method using electrical bias to directly nucleate and grow diamond films on substrates using a CVD approach wherein an electrical bias is applied to the substrate to enhance nucleation. Unfortunately, the high cost of CVD diamond ($5-$20/carat) restricts this approach.
Chemical vapor deposition (CVD) of diamond can be conducted at atmospheric pressures. However, industrial economies of scale cannot be realized with the manufacturing equipment currently available to produce large area CVD coatings. As a result, large area CVD coatings of diamond are too costly for manufacturers to feasibly employ.
U.S. Pat. No. 3,338,732 to Holcomb teaches a method of embedding silica particles in a porcelain enamel to give the porcelain enamel a rough appearance. A porcelain ground coat is applied to a metal substrate and heated so that the porcelain ground coat becomes fused to the metal substrate. Next, a wet cover coat of porcelain enamel is applied to the ground coat by spraying or dipping. While the cover coat is still wet, silica particles are sprinkled over the wet cover coat so that the silica particles become partially embedded in the wet cover coat. The cover coat is heated to fuse the cover coat to the ground coat and to fuse the silica particles to the cover coat. Next, a top coat is applied to the exposed portions of the cover coat and the silica particles. The top coat is then heated to fuse the top coat to the exposed portions of the cover coat and the silica particles. The resulting product has a rough surface on which there is no exposed silica.
U.S. Pat. No. 3,650,714 to Farkas discloses a method of coating single diamond chips with titanium or zirconium. The diamond chips are relatively large, having a mesh size of 200-250. Farkas teaches overlaying the titanium-coated diamonds with a second layer of nickel or copper or both nickel and copper, placing the coated diamonds on a steel substrate, and covering the coated diamonds with powdered ceramic. The powdered ceramic is then vitrified to secure the coated diamonds to the steel substrate. Since the diamond chips are coated, no portion of the surface of the diamonds is exposed and the diamonds are not bonded to the ceramic.
U.S. Pat. No. 4,749,594 to Malikowski et al. discloses a method for coating ceramic surfaces with hard substances. The method includes the steps of coating a metal or ceramic substrate with a metal powder, the spraying the substrate with a diamond powder, and heating the coated substrate to between 900 and 1200.degree. C. The heating step causes the metal powder to melt. The molten metal wets both the diamond particles and the substrate. When the melted metal is cooled, diamond particles from the diamond powder are integrally bonded to the substrate by the formed solder (hardened metal). The step of heating the coated substrate is performed in a high purity argon atmosphere or at sub-atmospheric pressure to prevent reactions between the active component of the metal powder and the remaining gases. In a first example, the invention is practiced with 40-50 micron diamond powder; in a second example, the invention is practiced with 60-80 micron diamond powder.
U.S. Pat. Nos. 5,164,220 and 5,227,940 to Caballero disclose a method of coating a metal substrate with treated diamond particles. Chemical plasma deposition is performed to grow an SiC crystal layer on the diamonds. Caballero discloses several different techniques for coating the metal substrate with the treated diamond particles. These techniques include sintering, brazing, and electroplating.
U.S. Pat. No. 5,453,303 to Holcombe et al. teaches a method of depositing a full diamond coating on a substrate. A powder mixture of glassy carbon and diamond particles is heated and impinged upon the substrate at high velocity. Upon impact with the substrate, the glassy carbon is phase transformed to diamond. The diamond particles promote the phase transformation of the glassy carbon to diamond. Similarly, U.S. Pat. No. 5,635,254 to Holcombe et al. teaches a method of depositing a layer of diamond or diamond-like material on a substrate. In one embodiment, a plasma gas stream heats and propels a mixture of glassy carbon and diamond particles onto the substrate. The mixture is then quenched on the surface of the substrate to produce a diamond coating.